The neurotoxic drugs 5,6-dihydroxytryptamine and 5,7-dihydroxytryptamine are currently in wide use in pharmacological laboratories because of their ability to destroy specific cells in the central nervous system. The cells destroyed belong to specific cell groups having a particular neurotransmitter. At present, the mechanism of action of the neurotoxic drugs is unknown. Evidence reported from this laboratory suggests that specificity is a matter of uptake by the cell. The drugs may be generally toxic to all cells provided they penetrate to the interior. The purpose of the present investigations was to determine the mechanism of toxicity and describe it at a molecular level if possible. It was thought that the toxicity might be related to the formation of quinones. If so, the site of a generalized toxicity might be mitochondria, in which the transport of electrons from NADH is inhibited by certain quinones, such as rotenone. This idea served as the starting point for the experiments to be described.